Friday Rocks #43: Mega Ptygmatic Folds

Mega ptygmatic folds
Mega ptygmatic folds

The above photo is taken from a field trip in 2010 to Death Valley and surrounding mountains / desert. This stop was Monarch Canyon. We (the students) are standing at a cliff edge, looking across a gorge at the opposing cliff wall. That’s the best scale I can give you for this. It’s huge, it’s awesome.

Friday Rocks #40: Pseudotachylyte Injectition

Pseudotachylyte injection.
Pseudotachylyte injection in the Homestake Shear Zone, Colorado

Fault pseudotachylyte is formed during an earthquake. The coseismic slip on the fault frictionally heats the surrounding rock and forms a melt. Rapid cooling of the melt forms a glass-like, dark rock, pseudotachylyte. A combination of the dynamic stresses imparted on the wall rock during the earthquake and dynamic pressurization will allow the melt to enter fractures in the wall rock.

In the photo above pseudotachylyte is the thin, black rock. The injection is at a 90 degree angle with the main pseudotachylyte vein.

Friday Rocks #39: Soft Sediment Deformation and Flame Structures

Soft sediment deformation at Point Lobos, California
Soft sediment deformation at Point Lobos, California

This weeks photo comes from a spectacular outcrop at Point Lobos, California. If you are there, try to time it with low tide, or you’ll miss this.

Soft sediment deformation occurs in unlithified sediments. Sometimes a trigger such as rapid loading by a mass wasting event or an earthquake is necessary to cause the deformation.

 

Flame structures (middle right in the photo) form when the overlying sediment (orange) is denser than the underlying sediment (black). This causes the overlying sediment to sink down into the low sediment, which pushes the lower sediment up. The structures formed resemble flames, hence the name.

Also in the photo (near the pointing finger) is a more competent sediment that was normal faulted as the unlithified sediment below it deformed. What other structures do you see in the photo?

Here’s another shot of the flame structures.

Point Lobos Flame Structures
Point Lobos Flame Structures

Friday Rocks #38: Folded Chert

I had to dig this one deep out of my archives, so sorry for the poor image quality.

Folded chert in the Marin Headlands
Folded chert in the Marin Headlands

This is a roadcut in the Marin Headlands, North of San Francisco California. Chert is made of microcrystalline quartz from radiolaria, microscopic protozoa with silicate skeletons. When radiolaria die their skeletons sink to the bottom of the ocean floor. After compaction and diagenesis, chert is made.

The chert in the Marin Headlands is part of an accretionary wedge block and is folded into chevron folds.

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Wahrhaftig, Clyde. “Structure of the Marin Headlands block, California: A progress report.” (1984): 31-50.

Friday Rocks #37: Porphyroclastic Ultramylonite

Feldspar porphyroblasts in the ultramylonite of the Pofadder Shear Zone
Feldspar porphyroclasts in the ultramylonite of the Pofadder Shear Zone

Porphyoclasts are deformed crystals found in a metamorphic rock. In a mylonite weaker minerals deform though crystalplastic processes and form a “toothpaste” texture. Stronger minerals will try to resist the deformation, but may break and stretch along with the weaker minerals. As the weak minerals recrystallize, they will flow around the remnant stronger minerals.

Feldspathic porphyclast in the ultramylonite of the Pofadder Shear Zone
Feldspathic porphyclast in the ultramylonite of the Pofadder Shear Zone

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Friday Rocks #36: Conjugate Deformation Bands

Conjugate Deformation Bands
Conjugate Deformation Bands

Deformation bands are mm wide shear offsets that typically occur in  porous rock such as sandstone at shallow depths (Aydin 1978). Offsets on deformation bands are typically small and may be distributed across the entire length of the structure (cm to kms). Brittle deformation via breaking and crushing of grains creates deformation bands which are filled with a gouge or cataclasite. Deformation bands have important implications for reservoir permeability and will typically host a cement.

Fractures that are at angles less than 90 degrees to each other are called Conjugate fractures. The orientation of fractures and offsets may be used to determine the stress state in the rocks during deformation (Anderson 1942, Jaeger and Cook 1969, Olsson et al. 2004). Assuming the above photo is a 2-D example, how do you think Sigma_1 and Sigma_3 are oriented?

Anderson, E.M., 1942. The Dynamics of Faulting and Dyke Formation with Application to Britain, Oliver and Boyd, Edinburgh, London.
Aydin, Atilla. “Small faults formed as deformation bands in sandstone.” Pure and Applied Geophysics 116.4-5 (1978): 913-930.
Jaeger, J.C., Cook, N.G.W., 1969. Fundamentals of Rock Mechanics, 2nd
ed, Chapman and Hall, London
Olsson, William A., John C. Lorenz, and Scott P. Cooper. “A mechanical model for multiply-oriented conjugate deformation bands.” Journal of Structural Geology 26.2 (2004): 325-338.

Friday Rocks #35: Asbestos in Serpentinite and Blueschist

Asbestos in Serpentine and Blueschits
Asbestos in Serpentinite and Blueschist

The rock in this photo as three components. Serpentinite (Green-yellow in photo), blueschist (blue-grey in photo), and asbestos (white-green fibrous vein in photo). Blueschist forms through the metamorphism of basalt at high pressure and low temperature. Serpentinite is a metamorphic rock formed in a low pressure environment through a reaction between ultramafic rock from the Earth’s mantle and water. The reaction is called “serpentiniztion” The asbestos mineral is often associated with serpentinite.

I found this rock when scrambling at the bottom of a cliff on the Big Sur coast of California. There were many small faults which sheared and mixed the different rocks types.